1. Field of the Invention
The present invention is directed to DC--DC switched mode power converters utilizing buck-type converter circuits.
2. The Background
Switched mode DC--DC power converters are common in the electronics industry. They are frequently used to convert one available DC level voltage to another DC level voltage, often needed for a particular set of semiconductor chips.
Such power converters generally use one or more electrically controlled switches (such as N- or P-Channel MOSFETs) the gates of which are controlled by a switched mode power supply controller circuit which is often integrated onto a single chip.
As electronic devices become faster, smaller and more portable, the need for increased electrical efficiency in DC--DC converters used in these devices is becoming more important. Energy wasted in portable electronics devices prematurely drains the battery powering the device and creates waste heat which must be managed. Relatively small increases in overall electrical efficiency--such as from 75% to 85%--result in a major decrease in wasted power and waste heat--e.g., from 25% to 15%.
Turning now to FIG. 1A, a buck converter circuit known in the art is shown. An input voltage is applied between the input terminals denoted Vin and GND. GND may be any fixed potential such as 0 volts DC relative to Vin. An input capacitor C1 preferably filters the input. Switch Q1 which is shown as an N-Channel MOSFET conducts current from Vin to node 10 when the gate of Q1 is powered by switching signal SS on line 12 from switched mode controller 14. Switched mode controller 14 may itself be powered from Vin and GND or another convenient power source. SENSE input on line 16 provides an indication of the output voltage Vout to controller 14 so that it may adjust the duty cycle of the switching signal SS to adjust Vout to a pre-programmed output voltage. The preprogrammed output voltage may be set in a number of ways known to those of skill in the art, such as with external components (not shown), built-in components, etc.
Diode D1 operates to restrict a charging current to flow from node 10 through inductor L1 rather than to ground (GND). The duty cycle of SS therefore controls Vout. Buck converters are known to be very efficient when the output voltage and the input voltage are relatively close and the duty cycle of the switching signal is high. Under these conditions the conduction angle is maximized and commutation losses are small relative to the power transmitted.
Where a relatively large voltage has to be bucked down to a relatively small voltage (e.g., 24 VDC to 3.3 VDC) the switching duty cycle is very small. This is illustrated in FIG. 1B which shows the voltage at node 10 of FIG. 1A over time under these circumstances. In this case the transitions are at relatively high voltage, high current and the efficiency becomes low. Thus where the switching duty cycle is less than about 50%, it is inadvisable from an efficiency point of view to use the buck circuit of FIG. 1A.
The single-ended forward circuit of FIG. 2 provides a solution. In the circuit of FIG. 2 transformer T1 utilizes a conventional turns ratio approach to step down the voltage on the secondary from that of the primary, much like in an AC circuit. This approach provides the flexibility of the turns ratio of the transformer and permits high duty cycles with low current commutations. It has a number of drawbacks, however. First, the circuit is made more complex and expensive by the presence of the transformer T1. It would be desirable to have a circuit not requiring a transformer for cost, size and complexity reasons. Second, the transformer in these sort of pulsed DC applications becomes saturated and requires time to recover. This, in turn, means that there is no power transfer during the recovery period, which requires larger, more expensive components to transfer the energy within a shorter time. It also results in the application of a voltage larger than Vin to the transistor Q1 which means that more expensive higher voltage transistors are required.
Turning now to FIG. 3, a buck converter using synchronous rectification is shown in schematic. In this synchronous version of the circuit of FIG. 1A, two switches, operated out of phase with one another (SS1 and SS2 are preferably 180 degrees out of phase with one another) are used instead of a switch and a diode. This provides a modest efficiency gain because the voltage drop on Q2 (FIG. 3) can be reduced over the voltage drop of D1 (FIG. 1A), say from 0.5V to 0.1V. The circuit also suffers, however, when low duty cycle switching is used to accomplish large voltage reductions. Accordingly, it would be desirable to provide a more efficient circuit for DC--DC power conversion of relatively large input to output voltage ratios without using a transformer.